1. Field of the Invention
This invention relates to an adhesive which is used to attach a thin plate to a board, and to a method for attaching a thin plate to a board. This invention particularly relates to an adhesive and a method which are used to attach a chip portion to a reinforcing plate in a process for manufacturing non-contact IC cards. Moreover, the present invention relates to a protective film that is used to process a chip portion thin, and a method for processing a chip portion thin.
2. Description of the Related Art
Non-contact type IC cards have been used in a variety of fields, such as railroad ticket gates, and are expected to be applied in a lot of fields in the future. A non-contact type IC card has an antenna for communication, a semiconductor chip (IC chip), and a sheet material to cover the antenna and the chip. An IC chip includes a chip portion and a reinforcing plate, which is normally made of stainless steel, to support the chip portion. The chip portion has a substrate made of silicon etc., and laminated films which are stacked thereon to form a circuit. The reinforcing plate has a function of protecting the chip portion against excessive deformation, and also has a function as a heat sink which absorbs heat that is generated by the chip portion.
Various methods for manufacturing an IC chip have been proposed. According to one method, first, a wafer, on which a plurality of chip portions are formed, is processed so that it is thinner from the back side of the wafer. Next, the thin wafer is diced into chip portions. Then, the separated chip portion is attached to a reinforcing plate in order to manufacture an IC chip. In order to attach a separated chip portion to a reinforcing plate, first, a plurality of reinforcing plates are held on a base, with the sides on which an adhesive is to be applied facing upward. Next, an adhesive is applied on the surface on which the adhesive is to be applied, and a chip portion is attached to the surface on which the adhesive has been applied. Thermosetting resins, such as epoxy resin, are often used as an adhesive. An adhesive made of a thermosetting resin is cured by heating the adhesive at a predetermined temperature and for a predetermined time in that state, and the IC chip is bonded to the reinforcing plate. For an epoxy resin, the predetermined time is 2 hours, and the predetermined temperature is 120 degrees centigrade.
In order to provide an adhesive with thermal and electrical conductivity after curing, an adhesive may contain a filler, as well as base material such as an epoxy resin, and a hardening agent to produce a rapid cure. A spherical filler having a diameter of 5 μm or less, and a scale-shaped filler having a thickness of 5 μm or less are disclosed in the specification etc. of Japanese Patent Laid-open Publication No. 2000-136341. A filler which has scale-shaped graphite powder having a maximum particle diameter of 100 μm or less and which has spherical copper powder that is covered with carbon and that has a maximum particle-diameter of 20 μm or less is disclosed In the specification etc. of Japanese Patent Laid-open Publication No. 2000-3987. A filler which has silica powder with a maximum particle diameter of 1-20 μm and which has a hydrophobic super fine silica powder with an average particle size of 2-50 nm is disclosed in the specification etc. of Japanese Patent Laid-open Publication No. 59158/93. Silica powder which has an average particle diameter of 1-20 μm and a maximum particle diameter of 50 μm or less is disclosed in the specification etc. of Japanese Patent Laid-open Publication No. 25512/94.
However, there are the following drawbacks in prior art. Specifically, thermosetting resins, which are often used as an adhesive for an IC chip, tend to gradually contract, i.e., tend to experience volumetric contraction, as curing progresses. FIGS. 1A, 1B show sectional views of a laminated structure consisting of a chip portion, a reinforcing plate, and an adhesive, when a chip portion is attached to a reinforcing plate according to prior art
As shown in FIG. 1A, adhesive S is applied to surface H1 of flat reinforcing plate H. and chip portion C is attached to upper surface S1 of adhesive S. Reinforcing plate H, adhesive S, and chip portion C form layers that are parallel to each other and that do not have out-of-the-plane strain, immediately after chip portion C is attached to adhesive S, as shown in FIG. 1A. However, adhesive S begins to produce volumetric contraction in accordance with polymerization of adhesive S and curing that follows. This means that the surface of adhesive S is forced to shrink. As the adhesive strength of adhesive S increases with time, adhesive S and reinforcing plate H, as well as adhesive S and chip portion C, are bound to each other.
In general, chip portion C tends to be deformed because the surface area of chip portion C is thin, and thereby there is limited stiffness. Therefore, bonding surface C1 of chip portion C, which is in contact with upper surface S1 of adhesives S, contracts when adhesive S suffers a volumetric (an areal) contraction with the force that is shown by arrow A in FIG. 1B. As a result, chip portion C is deformed so that it shrinks inwards. On the other hand, surface H1 of reinforcing plate H also contracts by adhesive S with the force that is shown by arrow B. However, reinforcing plate H is less subject to deformation than chip portion C because it is stiffer compared with that of chip portion C. Consequently, bonding surface C1 of chip portion C tends to shrink more than surface H1 of reinforcing plate H. In other words, the stacked structure consisting of chip portion C, reinforcing plate H, and adhesive S is bent such that chip portion C is bent with a smaller curvature than reinforcing plate H, or such that chip portion C is bent to form a convex, and the surface of reinforcing plate H to which adhesives S is not applied is bent to form a concave. Since such strain occurs in two directions in the plane of the stacked structure, chip portion C is in a stress state in which it is compressed from the circumferential portion toward the central portion, and chip portion C is deformed to form a concave in the central portion of chip portion C.
The strain that is induced in the chip portion in this way may cause undesired internal stress in the chip portion, may cause cracks that arise from the stress, and may promote extension of existing cracks. In particular, when an IC chip is incorporated into a non-contact IC card, the IC chip frequently experiences bending deformation during use. As a result, cracks may further extend, and, in the worst case, may cause breakage of the IC, resulting in considerable deterioration of reliability of IC chips.
Generally, adhesive strength that is required for an adhesive varies depending on the use. The requirement for adhesive strength is not so high for a chip portion that is used as an IC chip for a non-contact IC card, because the chip portion is often formed very thin in order to prevent thickness in a finished product. Moreover, an IC chip is covered with and fixed to a protective film, and is finally protected with a coating sheet. For these reasons, the requirement for adhesiveness is not so strict.
However, if an IC chip has a residual strain and is processed thinner, then the chip is more apt to experience cracks because of bending force in a reinforcing plate. Furthermore, if the bonding process is performed improperly, then an IC chip tends to experience chipping and cracks on the surfaces. Therefore, it is very important to prevent bending deformation, which is caused by volumetric contraction of the adhesive, in a chip portion of an IC chip that Is used as a non-contact IC card.
There also exist similar problems in various fields of IC chips other than the field of non-contact ICs. For example, ICs, when applied to cellular phones, need compact and high-density mounting in three-dimensional arrangements using methods such as piggyback stacking. However, such an arrangement tends to cause strain due to the stress in an adhesive during the process for stacking, and the strain that is caused may affect the characteristics of an element. It is reported that an IC chip is more apt to suffer damage due to the influence of mechanical external force. Furthermore, there is the problem that the durability of an IC chip is deteriorated by an increase in temperature of an element that is caused by the decrease in thermal conductivity of an adhesive.
Moreover, ICs, when they are used as sensors, are requested to be processed thin in order to secure stable mounting, as well as to reduce the size of the IC chip. However, the element characteristics may be affected by the strain that is caused by a stress in an adhesive. Furthermore, a non-uniform adhesive may cause an increase and variation in temperature because of local heat buildup. In this case, it is reported that an offset drift may occur and stability may be spoiled.
Furthermore, there is another problem regarding thin wafers during the process of dicing a wafer. In general, in order to dice a wafer into chips, a protection tape is applied to the wafer by means of an organic resin, and the wafer is diced in that state. Such a method can prevent chipping during dicing and mitigate mechanical damage on an element. Since chipping tends to occur more frequently as a wafer becomes thinner, the necessity for a protection tape increases as a wafer becomes thinner. However, the protection tape, which may reduce thermal conductivity, if it remains on the wafer after the wafer is diced into chip portions, needs to be removed prior to attaching a reinforcing plate to the chip portion, leading to increase in steps and a reduction in yield.
Therefore, even if certain protection material is provided in order to prevent chipping, it is desirable that the protection material is used as a part of the product in order to allow easy handling of the separated pieces. In order to do so, it is desirable that the stress in protective layer due to contraction after curing is small so that a chip portion is not curved or does not experience strain after it is attached to a reinforcing plate, and, in addition, it is also desirable that the reduction in thermal conductivity and non-uniformity are limited.